Long Term Evolution (hereinafter referred to as LTE) is a next-generation mobile broadband network standard defined in the Third Generation Partnership Project (3GPP). It uses orthogonal frequency division multiplexing (OFDM) and introduces multiple-input multiple-output (MIMO) technology, while it can support 1.25-20 MHz bandwidth, which greatly increases the peak data rate and the system capacity, and supports the peak rates of 100 Mbit/s in downlink and 50 Mbit/s in uplink within the 20 Mhz bandwidth; its flat network architecture enhances the scheduling and radio resource control efficiency, and shortens the connection delay.
The Multimedia Broadcast/Multicast Service (MBMS) has been supported in the third generation mobile communication system, and the Enhanced Multimedia Broadcast/Multicast Service (eMBMS) architected in the fourth generation mobile communication LTE system is taken as a business with strong applicability in the industry. Its implementation mode is to receive digital audio/video services in the form of broadcast/multicast by an intelligent terminal with video capability through the physical multicast channel (PMCH). For the eMBMS service implementation and networking mode, the 3GPP protocol standard gives a complete solution.
With the rapid development of Internet and the popularity of large-screen multi-function mobile terminals, there have been a lot of mobile data multimedia services and a variety of high-bandwidth multimedia services such as video conferences, television broadcasts, video-on-demand, advertisements, online education, and interactive games, which meets the mobile users' demands on multiple services on one hand, and brings new service growth point for mobile operators on the other hand. These mobile data multimedia services require multiple users to simultaneously receive the same data, which, compared to the general data services, has features of a large amount of data, long duration, and delay-sensitive.
In order to use the mobile network resources effectively, the 3GPP proposes the Multimedia Broadcast Multicast Service (MBMS). The MBMS service is a technology of sending data from one data source to a plurality of target mobile terminals, so as to achieve network (including the core network and the access network) resource sharing, and improve the network resource (particularly air interface resource) utilization. With the MBMS service defined in the 3GPP, not only the multicast and broadcast of messages with low speed and plain text can be achieved, but also the multicast and broadcast of multimedia services of high speed can be achieved, and a variety of video, audio and multimedia services are provided, which undoubtedly conforms to the trend of the future mobile data development and provides a better business prospect for the 3G development.
The enhanced Multimedia Broadcast Multicast Service (eMBMS) introduces the MBMS over a Single Frequency Network (MBSFN) transmission mode in the access network. According to the current 3GPP protocol, one MBSFN area comprises a plurality of cells, these cells are configured with the same MBSFN resources, and all the cells in one MBSFN area send the same MBMS service in these MBSFN resources. Using this transmission mode can save frequency resources and improve spectrum efficiency. At the same time, the diversity effect brought by this multi-cell co-frequency transmission can solve problems such as blind spot coverage, enhance the reception reliability, and improve the coverage rate.
The user equipment (UE, or called terminal) receiving the MBMS service can be in one of the following two states: one is RRC_CONNECTED (referred to as connected state); the other is RRC_IDLE (referred to as idle state). According to whether the UE is currently receiving the unicast service or not, whether the UE which is receiving the MBMS service is in the connected state or the idle state can be judged; when the UE is receiving the unicast service, the UE is in the connected state; the UE in the idle state does not establish a radio resources control (RRC) connection with the network side, while the UE in the connected state establishes an RRC connection with the network side.
According to the 3GPP protocol, the MBSFN area division generally has the following modes:
1. for a certain MEMS service, the MBSFN area is divided based on a service area of the service, as shown in FIG. 1;
By this way, the advantage is that, for a certain MBMS service, it only needs to broadcast the MBMS in the desired area, and the resource management is relatively reasonable. The downside is that, if the service areas of some services (such as: some area information, shopping advertisements, and so on) are relatively small or not continuous, thus it cannot adequately improve a receiving gain at the boundary of the cell.
2. an MBSFN area is established on a physically continuous area, as shown in FIG. 2;
By this way, the advantage is that, by establishing a MBSFN area in a physically continuous area, it avoids the interference between the MBSFN boundaries and increases the receiving gain at the boundary of the cell. For an area that is physically discontinuous, it does not need to consider the interference between the adjacent MBSFNs and there is no receiving gain, thus different MBSFN areas can be set.
3. one MBSFN is established in an entire network, as shown in FIG. 3.
By this way, the advantage is that the implementation is simple and the receiving gain of the UE at the boundary of the cell can be increased, and the UE has a good movability performance within the entire network.
However, according to the existing modes, no matter which one of the above three modes by which the MBSFN area is divided, for a certain MBMS service, since the location of the user equipment cannot be known, if the MBMS is broadcasted in the entire service area, if the service area is too large, and the number of the UEs is quite few or the distribution is relatively concentrated, the waste of radio resources is serious.
The 3GPP specifies that eight different MBSFN areas which are supported maximally overlay the same area, and each MBSFN area maximally supports 16 physical multicast channels (PMCH), each PMCH maximally support 29 multicast traffic control channels (MTCHs). Therefore, according to the limitations on the protocol field, the number of eMBMS sessions simultaneously supported in one cell in theory is 3480. However, due to the limitations of air interface resources, it is actually unattainable.
Mobile TV is an eMBMS based service application. Taking a mobile TV as an example to analyze, a certain city provides a 256 kbps-mobile TV service, time-division Long Term Evolution (TD-LTE) network, 20M system bandwidth, 5 ms-radio frame, and 2:2-configuration. Two eMBMS subframes can be used maximally in 10 ms. With the quadrature phase shift keying signal (QPSK) adjustment mode, it can provide about 4 Mbps data traffic in downlink, that is, about 16 channels of programs, which basically cannot meet the program capacity requirements of one city. The main reason which causes this capacity limitation is mainly that the existing ways for managing air interface resource are extensive, and in many areas, there is no terminal for receiving, and it still needs to occupy the air interface resources to broadcast. Especially when there are only few terminals receiving a certain program, it also needs to broadcast in the entire city, which largely wastes the resources.
For a certain MBMS service, once the service is sent in a certain MBSFN area, this service needs to be sent in all the cells covered by the MBSFN area. If the MBSFN area is very large, some parts (including a plurality of cells) of this area do not have one terminal that receives or is interested in receiving the service, which results in:
1) some parts of the entire MBSFN area do not have a terminal to receive, and the air interface resources still need to be occupied to broadcast, which wastes the radio resources;
2) the some parts of the area also do not have idle resources to send other MBMS services which other terminals are interested in. For example, the MBMS service 1 is transmitted in a certain MBSFN area, and there is no terminal in some part of this area interested in receiving the MBMS service 1, even though the terminals in the some part are interested in the MBMS service 2, but since the radio resources in these parts are occupied by the MBMS service 1, there are no idle resources to send the MBMS service 2.
In the LTE, a tracking area identity (TAI) is used to identify the tracking area (TA), similar to the concept of location area (LA) in the 2G/3G circuit-switched (CS) domain, or the concept of routing Area (RA) in the packet-switched (PS) domain, since there are only PS domains in the LTE, there is only the concept of TA. In fact, the LTE uses the TA list to track the location of the UE at the network side, and supports the idle state mobility (such as re-scheduling) and paging.
The tracking area update (TAU) is a TA update process. That is, when the location of a user changes and it needs to update the TA list (that is TAI), there are a number of signaling interactions with the network side.
The functions implemented by the TAU comprise:
the UE enters into a new TA, and updates the UE's current location to the network side;
updating is performed periodically, the UE periodically reports the current location;
the UE capability changes and is updated to the network side;
when the bearers in the Evolved Packet System (EPS) change, the network side is notified of which bearers are still in the activated state.
The UE may send the MME a tracking area update request message to start a TAU process.